#include <iostream>
#include <cstdio>
#include <cmath>
using namespace std;
int main()
{
    double a, b, c, p;
    cin >> a >> b >> c;
    p = 1.0 / 2.0 * (a + b + c);
    printf("%.1f", sqrt(p * (p - a) * (p - b) * (p - c)));
}